Microstructure and mechanical properties of nanocrystalline titanium and Ti-Ta-Nb alloy manufactured using various deformation methods

Mechanical properties and TEM microstructure studies have been carried out of nanocrystalline titanium, Ti10Nb10Ta and Ti10Nb obtained by various technological routes, including: powder metallurgy (ball milling and hot pressing), Equal Channel Angular Pressing (ECAP), hydroextrusion (HE) and high pressure torsion (HPT). The HE processed material in the form of 20 mm rods was extruded at a strain rate of 2.5 x 10(2) s(-1) to a diameter of 3 mm, which corresponds to the true strain of 3.8. Resulting Yield Strength (YS) at the crystal size below 80 nm exceeded 1000 MPa, i.e. attained a value of 3 times more than the initial material. Equal-Channel Angular Pressing (ECAP) at 723 K was applied to produce nanostructured titanium. Grain refinement was observed already after one pass (considerable number of grains with d < 100 nm was noted). It was accompanied by a growth of strength and slight decrease in the elongation. ECAP processing up to 4 passes resulted in further slight growth of strength and further slight loss of elongation. The titanium powder prepared by ball milling in a high energy mill decreased its crystal size down to 10 nm and reached microhardness HV20 = 1000. The additions of Nb and Ta resulted in a similar grain refinement but lower hardness. Uniaxial hot pressing at 650 degrees C, followed by vacuum annealing resulted in similar microhardness as for powders. TEM studies performed using quantitative metallography allowed to estimate mean grain size at 150 nm. HPT technique at the pressure of 5 GPa resulted in finest grain size as compared to other preparation techniques leading to nanoscale grain refinement in Ti samples. The mean crystal size was estimated at about 30 nm. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.